166 THE BRITISH NATURE BOOK 



almost immediately. Notice (under the microscope) the remarkable single 

 eye. 



2. The Cyclops is, as its classical name implies, a one-eyed crustacean. It is 

 a tiny, pear-shaped creature, with long antennae and four pairs of legs. The 

 male is smaller and much rarer than the female, which may always be identi- 

 fied by the two egg-bags which she carries attached to her abdomen, contain- 

 ing fifty or sixty eggs apiece ! The larva when hatched is almost invisible, 

 and is not in the least like its parents until it assumes the adult form. 



3. The Fresh-water Shrimp (Gammarus pulex) is also found in every pond 

 and stream. It feeds on decaying animal matter, and is a good scavenger 

 for an aquarium. The eggs are carried by the female in her front legs ; and 

 the young, on hatching out, remain with their mother until scattered. 



Fig. 3 in the illustration is another Water-flea, Cypris, an insect with 

 a bivalve shell, about the size of a grain of millet, and not unlike a miniature 

 mussel shell. It possesses a single eye and two pairs of antennae. A male 

 Cypris has never been found. 



Fig. 4 is the Water-louse, Asellus aquaticus, a scavenger to be found in 

 the mud and on weeds. Its breathing apparatus, a series of flat appendages 

 beneath the abdomen, is worth observing. 



4. The Crayfish (Astacus fluviatilis) is to be found in many of our streams, 

 especially in limestone districts. These creatures make a burrow in the bank or 

 find a natural crevice, in which they reside for the winter ; but in warmer weather 

 they emerge much more frequently. They eat all kinds of food, not only 

 vegetable, but animal snails, meat, fish ; they will even devour one another, 

 and I have photographed crayfish engaged in a tremendous battle for 

 supremacy. 



The female lays eggs in November and December, and carries them fixed 

 to her abdominal legs till they hatch in the following June. The Crayfish 

 moults that is, casts off its outer shell eight times in the first year, five times 

 in the second, twice in the third, and once a year for the rest of its existence. 

 This is accounted for by the fact that, as the creature grows, the shell does 

 not grow with it, and therefore has to be discarded when too small, and a new 

 one formed. 



Marine Crustaceans. 



i. The Barnacle. The most familiar species is the Acorn Barnacle, or ACORN 

 SHELL (Balanus), found on almost every rock on our coasts, and sharp enough 

 to cut hands and feet as we clamber about. Inside the cone-shaped shell is a 

 small creature belonging to the Cirripedia, or Curl-footed Crustaceans. When 

 first hatched it swam about freely, a minute creature with three pairs of legs, 

 one eye, and a delicate shell on its back ; but after moulting several times 

 it settled down on a rock, a shell, or a piece of wood, glued itself firmly to its 

 support by means of suckers, and gradually assumed the acorn shape. These 

 Balani feed by protruding their six pairs of legs through the slit in the lid 



